1. Field of the Invention
The present invention relates to an image display apparatus and a method for driving the apparatus and, more particularly, to a display apparatus having capacitive light-emitting devices, such as organic electroluminescence devices, and the method for driving the apparatus.
2. Description of the Related Art
An electroluminescence display panel which has a plurality of organic electroluminescence devices arranged in a matrix form is receiving great attention as a display which can have lower power consumption and high display quality and can be suitable for thin-profile display apparatus. As shown in FIG. 1, the organic electroluminescence device has at least a single organic function layer 102, comprised of an electron-transport layer, a light-emitting layer, hole-transport layer, etc., and a metal electrode 103, both formed on a transparent substrate 100 like a glass plate on which a transparent electrode 101 is formed. As a positive voltage is applied to the anode of the transparent electrode 101 and a negative voltage to the cathode of the metal electrode 103, i.e., as a DC voltage is applied between the transparent electrode 101 and the metal electrode 103, the organic function layer 102 emits light. With the organic function layer formed of an organic compound which can be expected to have an excellent emission characteristic, the electroluminescence display can be used practically.
An organic electroluminescence device (hereinafter also referred to as xe2x80x9cEL devicexe2x80x9d) can be expressed as an electrically equivalent circuit as shown in FIG. 2. As apparent from the circuit diagram, the device can be replaced with a capacitive component C and a component E with a diode characteristic that is coupled in parallel to the capacitive component C. The EL device is thus a capacitive light-emitting device. When a DC drive voltage is applied between the electrodes of the EL device, charges are stored in the capacitive component C. When the drive voltage exceeds the barrier voltage or emission threshold value inherent to the device, a current starts flowing into the organic function layer that has the light-emitting layer from one of the electrodes (the anode side of the diode component E) and light is emitted with the intensity proportional to the current.
The voltage V v.s. current I v.s. luminance L characteristic of the device is similar to the diode characteristic such that the current I is very small for the voltage equal to or lower than the emission threshold value Vth but abruptly increases when the voltage becomes greater than the emission threshold value Vth, as shown in FIG. 3. The current I is approximately proportional to the luminance L. Such a device provides a luminance proportional to the current that accords to the drive voltage when the drive voltage above the emission threshold value Vth is applied to the device, but it has substantially no drive current flowing when the applied drive voltage is lower than the emission threshold value Vth, so that the luminance stays substantially equal to zero.
Passive matrix driving can be used to drive a display panel which uses a plurality of such EL devices. FIG. 4 exemplifies the structure of a passive matrix display panel. An N number of cathode lines (metal electrodes) B1 to Bn are laid horizontally, and an M number of anode lines (transparent electrodes) A1 to Am are laid in parallel vertically to cathode lines B1-Bn, with light-emitting layers of EL devices E1,1 to Em,n placed at (a total of nxc3x97m) intersections between the anode lines A1-Am and the cathode lines B1-Bn. The devices E1,1 to Em,n which serve as pixels are arranged in a grid pattern, and have their one ends (each of which corresponds to the anode of the diode component E in the aforementioned equivalent circuit) connected to the anode lines A1-Am at the respective intersections between the vertical anode lines A1-Am and the horizontal cathode lines B1-Bn and the other ends (each of which corresponds to the cathode of the diode component E in the equivalent circuit) connected to the cathode lines B1-Bn. The cathode lines B1-Bn are connected to, and driven by, a cathode-line scan circuit 1, while the anode lines A1-Am are connected to, and driven by, an anode-line driver 2.
The cathode-line scan circuit 1 has scan switches 51 to 5n which are associated with the cathode lines B1-Bn and respectively determine the potentials of the cathode lines B1-Bn. Each of the scan switches 51-5n connects either a reverse bias voltage Vcc (e.g., 10 V), which is a power supply voltage, or a ground potential (0 V) to the associated cathode line.
The anode-line driver 2 has current sources (e.g., constant current sources) 21 to 2m and drive switches 61 to 6m, which are associated with the anode lines A1-Am and supply the drive current to the respective devices via the respective anode lines. The anode-line driver 2 performs ON/OFF control on the drive switches 61-6m. to let the current flow through the respective anode lines A1-Am individually. It is typical to use current sources as the drive sources instead of voltage sources like constant voltage sources for reasons such as the aforementioned current v.s. luminance characteristic being stable with respect to a temperature variation whereas the voltage v.s. luminance characteristic is not. The amount of the current to be supplied from each of the current sources 21-2m is set to the amount that is necessary to keep the associated device emitting light at the desired instantaneous luminance (hereinafter this state will be called xe2x80x9csteady emission statexe2x80x9d). As electrical charges are being stored in the capacitive component C in the device while the device is in the steady emission state, the voltage across the device becomes a specified value Ve (hereinafter called xe2x80x9cspecified emission voltagexe2x80x9d).
The anode lines A1-Am are also connected to an anode-line resetting circuit 3, which has shunt switches 71-7m provided for the respective anode lines. As each shunt switch is selected, the anode-line resetting circuit 3 sets the associated anode line to the ground potential.
The cathode-line scan circuit 1, the anode-line driver 2 and the anode-line resetting circuit 3 are connected to an emission controller 4.
In accordance with image data supplied from an image data generating system (not shown), the emission controller 4 controls the cathode-line scan circuit 1, the anode-line driver 2 and the anode-line resetting circuit 3 to display images carried by the image data. The emission controller 4 controls switching of the scan switches 51-5n to send a scan-line selection control signal to the cathode-line scan circuit 1, select one of the cathode lines that corresponds to the horizontal scan period of the image data, connect the selected cathode line to the ground and apply the reverse bias voltage Vcc to the other cathode lines. The reverse bias voltage Vcc is applied by a constant voltage source to be connected to each cathode line in order to prevent cross-talk emission from the devices connected at the intersections of the driven anode lines and the cathode lines which are not selected for scanning. The reverse bias voltage Vcc is generally set equal to the specified emission voltage Ve. As the scan switches 51-5n are sequentially switched to the ground potential every horizontal scan period, the cathode line which has been switched to the ground potential serves as a scan line which permits the devices connected to the cathode line to emit light.
The anode-line driver 2 performs drive control on the selected scan line. The emission controller 4 generates drive control signals (drive pulses) indicating which device connected to the scan line should be enabled to emit light at what timing and for how long, in accordance with pixel information specified by the image data, and sends the drive control signal to the anode-line driver 2. In accordance with the drive control signal, the anode-line driver 2 implements ON/OFF control on some of the drive switches 61-6m and supplies the drive current to the devices corresponding to the pixel information via the associated anode lines A1-Am. Consequently, the devices supplied with the drive current emit light according to the pixel information.
The reset operation of the anode-line resetting circuit 3 is performed in response to a reset control signal from the emission controller 4. The anode-line resetting circuit 3 sets any of the shunt switches 71-7m which corresponds to the anode line to be reset that is indicated by the reset control signal, and sets off the other shunt switches.
Japanese Laid-Open Patent Publication (KOKAI) No. H 9-232074 of the same applicant as the present application discloses a driving method of executing a reset operation to discharge electrical charges stored in individual devices laid out in a grid pattern on a passive matrix display panel immediately before changing the scan line (this method will be hereinafter called xe2x80x9creset driving methodxe2x80x9d). The reset driving method quickens the rising of emission of devices at the time the scan line is changed over to another one. The reset driving method for a passive matrix display panel will now be described with reference to FIGS. 4 to 6.
The operation exemplified in FIGS. 4 to 6 is for a case where the cathode line B1 is scanned to permit the devices E1,1 and E2,1 to emit light, then scanning is shifted to the cathode line B2 to permit the devices E2,2 and E3,2 to emit light. For easier understanding of the description, the devices which are emitting light are indicated by the symbols of diodes, while the devices which are not emitting light are indicated by the symbols of capacitors. The reverse bias voltage Vcc to be applied to the cathode lines B1-Bn is 10 V, the same as the specified emission voltage Ve for the devices.
First, only the scan switch 51 is switched to the ground potential position and the cathode line B1 is scanned in FIG. 4. The reverse bias voltage Vcc is applied to the other cathode lines B2-Bn by the scan switches 52-5n. At the same time, the current sources 21 and 22 are respectively electrically connected to the anode lines A1 and A2 by the drive switches 61 and 62. The other anode lines A3-Am are switched to the ground potential (earth) position of 0 V by the shunt switches 73-7m. In the case of FIG. 4, therefore, only the devices E1,1 and E2,1 are biased in the forward direction, and the drive current flows into those devices from the current sources 21 and 22 as shown by the arrows, causing only the devices E1,1 and E2,1 to emit light. In this state, the devices E3,2 to Em,n which are not emitting light and are indicated by hatching are charged to the illustrated polarity.
The following reset control is executed immediately before scanning is shifted from the steady emission state in FIG. 4 to a state where the next devices E2,2 and E3,2 emit light. Specifically, as shown in FIG. 5, all the drive switches 61-6m are opened, all the scan switches 51-5n and all the shunt switches 71-7m are switched to the ground position, and all of the anode lines A1-Am and the cathode lines B1-Bn are temporarily shunt to the ground position of 0 V to be all reset. When the all-resetting is carried out, all of the anode lines and the cathode lines have the same potential of 0 V, so that the charges stored in the individual devices are discharged through the routes indicated by the arrows in FIG. 5. As a result, the charges stored in all the devices will vanish instantaneously.
After the charges stored in all the devices are set to zero, only the scan switch 52 corresponding to the cathode line B2 is switched to the 0 V position to scan the cathode line B2 as shown in FIG. 6. At the same time, the drive switches 62 and 63 are closed to connect the current sources 22 and 23 to the associated anode lines, and the shunt switches 71 and 74-7m are switched on to apply 0 V to the anode lines A1 and A4-Am.
As apparent from the above, the emission control in the reset driving method repeats the scan mode during which one of the cathode lines B1-Bn is set active and the following reset mode. The scan mode and reset mode are performed every horizontal scan period (1 H) of image data. If the state in FIG. 4 were shifted to the state in FIG. 6 directly without the reset control, the drive current to be supplied from the current source 23, for example, not only would flow into the device E3,2 but would also be used to cancel the charges of the opposite polarity (shown in FIG. 4) stored in the devices E3,3 to E3,n. It would therefore take time to render the device E3,2 in the steady emission state (to set the voltage across the device E3,2 to the specified emission voltage Ve).
Through the above-described reset control, however, the potentials of the anode lines A2 and A3 become approximately Vcc the instant scanning is shifted to the cathode line B2, so that the charge current flow into the devices E2,2 and E3,2 which should emit light next, through a plurality of routes from the constant voltage sources connected to the cathode lines B1 and B3-Bn as well as from the current sources 22 and 23. The charge current make the voltages across the devices E2,2 and E3,2 reach the specified emission voltage Ve instantaneously, thus enabling instantaneous transition to the steady emission state.
Since the conventional reset driving method temporarily resets all of the cathode lines and the anode lines by connecting those lines to the ground potential of 0 V or the same potential as the reverse bias voltage Vcc before emission control moves to the next scan line, it is possible to speed up charging of the devices to emit light in the next scan to the specified emission voltage Ve at the time the scan line is switched and quicken the rising of emission of the devices on the switched scan line which should emit light.
Since, in the passive matrix display panel employing the reset driving method, the charges stored in the parallel capacitive components of the devices that are to emit light are discharged before switching to the next scan line, however, it has a deficiency that consumption power is wasted. Paying attention to a case where the EL devices Em,1 and Em,2 connected to the anode line Am do not emit light when the scanning target is switched from the cathode line B1 to the cathode line B2 as shown in FIGS. 4 to 6, for example, the power loss of those devices will be explained referring to FIGS. 7A through 7C. As shown in FIG. 7A, while the device Em,1 is not charged during the first scanning of the cathode line B1 due to the cathode line B1 and anode line Am both being at the ground potential, the devices Em,2 to Em,n are biased in the reverse direction with the reverse bias voltage Vcc and their parallel capacitive components are charged with charges Q via the cathode lines B2-Bn. The total amount of charges of the devices on the anode line Am which are not emitting light becomes (nxe2x88x921)Q. Next, all-resetting to 0 V causes all the charges (nxe2x88x921)Q to be discharged to zero via the anode line A1 and cathode lines B2-Bn, as shown in FIG. 7B. During the second scanning of the next cathode line B2, as shown in FIG. 7C, the parallel capacitive components of the devices Em,1 and Em,3 to Em,n on the anode line Am are charged with charges (nxe2x88x921)Q. When one pays attention to the devices which do not emit light, therefore, wasteful discharging occurs every resetting operation. In other words, in a case where an anode line is reset between the first and second scans and the devices on that anode line, such as the devices E2,1 and E2,2 on the anode line A2, are rendered off from off, consumed power of charges 2(nxe2x88x921)Q is wasted. The power loss by the charging and discharging of the parallel capacitive components in a plurality of EL devices of the display panel becomes greater in proportion to the parallel capacitance per unit area and the effective area of the display panel. It is therefore necessary to reduce the power loss.
Accordingly, it is an object of the present invention to provide a display apparatus with capacitive light-emitting devices, which quickens the rising of light emission without increasing power consumption.
To achieve the object, according to one aspect of the present invention, there is provided a method for driving a display apparatus with capacitive light-emitting devices including a plurality of capacitive light-emitting devices located at a plurality of intersections of drive lines and scan lines and respectively electrically connected between the scan lines and the drive lines, scan switches for connecting the scan lines to one of a first potential and a second potential different from each other when activated, drive switches for connecting the drive lines to at least one of the first and second potentials or a drive source when activated, and emission control means for controlling the drive switches and the scan switches, whereby the drive switches are activated so as to selectively connect the drive lines to the drive source to allow selected capacitive light-emitting devices to emit light in synchronism with scan timings at which the scan switches connect the scan lines to a lower one of the first and the second potentials, comprises the steps of inserting a reset period between each of scan periods; selecting non-connection keeping drive lines among all of the drive lines which are not connected to the drive source in a previous scan period and a present scan period; and connecting all of the scan lines to the same reset potential, opening the selected non-connection keeping drive lines and connecting the other drive lines to the reset potential in the reset period.
According to a second aspect of the present invention, there is provided a method for driving a display apparatus with capacitive light-emitting devices including a plurality of capacitive light-emitting devices located at a plurality of intersections of drive lines and scan lines and respectively electrically connected between the scan lines and the drive lines, scan switches for connecting the scan lines to one of a first potential and a second potential different from each other when activated, drive switches for connecting the drive lines to at least one of the first and second potentials or a drive source when activated, and emission control means for controlling the drive switches and the scan switches, whereby the drive switches are activated so as to selectively connect the drive lines to the drive source to allow selected capacitive light-emitting devices to emit light in synchronism with scan timings at which the scan switches connect the scan lines to a lower one of the first and the second potentials, comprises the steps of inserting a reset period between each of scan periods; selecting unconnected drive lines among all of the drive lines which are not connected to the drive source in a present scan period; and connecting all of the scan lines to a same reset potential, opening the selected unconnected drive lines and connecting the other drive lines to the reset potential in the reset period.
In the method according to the present invention, selection of the non-connection keeping drive lines or the unconnected drive lines is carried out in a reset period immediately before the present scan period.
In the method according to the present invention, one of the first potential and the second potential is a ground potential, while the other one is a potential greater than a potential difference between a specified emission voltage of the capacitive light-emitting devices and an emission threshold voltage.
In the method according to the present invention, one of the first potential and the second potential is a ground potential, while the other one is substantially equal to a specified emission voltage of the capacitive light-emitting devices.
In the method according to the present invention, the reset potential is equal to one of the first and second potentials.
In the method according to the present invention, a scan line to which the selected capacitive light-emitting devices are connected is connected to the ground potential, and the other scan lines are connected to a potential greater than the potential difference between the specified emission voltage of the capacitive light-emitting devices and the emission threshold voltage.
In the method according to the present invention, a scan line to which the selected capacitive light-emitting devices are connected is connected to the ground potential, and the other scan lines are connected to a potential substantially equal to the specified emission voltage of the capacitive light-emitting devices.
In the method according to the present invention, drive lines other than that drive line to which the selected capacitive light-emitting devices to be connected to the drive source for light emission are connected are connected to the ground potential.
In the method according to the present invention, the capacitive light-emitting devices are electroluminescence devices.
In the method according to the present invention, the capacitive light-emitting devices are located at intersections of a plurality of drive lines extending approximately in parallel to one another and a plurality of scan lines extending approximately perpendicularly to the drive lines and approximately in parallel to one another and respectively electrically connected to the scan lines and the drive lines.
According to further aspect of the present invention, there is provided a display apparatus with capacitive light-emitting devices which includes a plurality of capacitive light-emitting devices located at a plurality of intersections of drive lines and scan lines and respectively electrically connected between the scan lines and the drive lines; scan switches for connecting the scan lines to one of a first potential and a second potential different from each other when activated; drive switches for connecting the drive lines to at least one of the first and second potentials or a drive source when activated; emission control means for controlling the drive switches and the scan switches in such a way that the drive switches are activated so as to selectively connect the drive lines to the drive source to allow selected capacitive light-emitting devices to emit light in synchronism with scan timings at which the scan switches connect the scan lines to a lower one of the first and the second potentials; and discrimination means for selecting non-connection keeping drive lines among all of the drive lines which are not connected to the drive source in a previous scan period and a present scan period, wherein the emission control means provides a reset period between the scan timings, and performs such control as to connect all of the scan lines to the same reset potential, to open the non-connection keeping drive lines selected by the discrimination means and to connect the other drive lines to the reset potential in the reset period.
According to another aspect of the present invention, there is provided a display apparatus with capacitive light-emitting devices which includes a plurality of capacitive light-emitting devices located at a plurality of intersections of drive lines and scan lines and respectively electrically connected between the scan lines and the drive lines; scan switches for connecting the scan lines to one of a first potential and a second potential different from each other when activated; drive switches for connecting the drive lines to at least one of the first and second potentials or a drive source when activated; emission control means for controlling the drive switches and the scan switches in such a way that the drive switches are activated so as to selectively connect the drive lines to the drive source to allow selected capacitive light-emitting devices to emit light in synchronism with scan timings at which the scan switches connect the scan lines to a lower one of the first and the second potentials; and discrimination means for selecting unconnected drive lines among all of the drive lines which are not connected to the drive source in a present scan period, wherein the emission control means provides a reset period between the scan timings, connects all of the scan lines to a same reset potential, opens the unconnected drive lines selected by the discrimination means and connects the other drive lines to the reset potential in the reset period.
In the display apparatus according to the present invention, selection of drive lines by the discrimination means is carried out in a reset period immediately before the present scan period.
In the display apparatus according to the present invention, one of the first potential and the second potential is a ground potential, while the other one is a potential greater than a potential difference between a specified emission voltage of the capacitive light-emitting devices and an emission threshold voltage.
In the display apparatus according to the present invention, one of the first potential and the second potential is a ground potential, while the other one is substantially equal to a specified emission voltage of the capacitive light-emitting devices.
In the display apparatus according to the present invention, the reset potential is equal to one of the first and second potentials.
In the display apparatus according to the present invention, in each scan period, the emission control means performs such control as to connect that scan line to which the selected capacitive light-emitting devices are connected, to the ground potential, and connect the other scan lines to a potential greater than the potential difference between the specified emission voltage of the capacitive light-emitting devices and the emission threshold voltage.
In the display apparatus according to the present invention, in each scan period, the emission control means performs such control as to connect that scan line to which the selected capacitive light-emitting devices are connected, to the ground potential, and connect the other scan lines to a potential approximately equal to the specified emission voltage of the capacitive light-emitting devices.
In the display apparatus according to the present invention, in each scan period, the emission control means executes such control as to connect drive lines other than that drive line to which the selected capacitive light-emitting devices to emit light are connected, to the ground potential.
According to the present invention, in the so-called reset driving method, not all the drive lines are set to the same potential in a reset period, but any drive line on which devices that are not emitting light are rendered off in the reset period between scanning period is extracted, i.e., selection of any one of all the drive lines which is not connected to a drive source and continuously emits no light in both the previous and present scan periods is determined, and the selected drive line is opened, so that the residual charges in the capacitive components of all the devices on that drive line can be held undischarged. Meanwhile, it is possible to avoid charging which does not contribute to light emission or charging of the non-emitting devices at the present scanning. It is therefore possible to provide a display apparatus with capacitive light-emitting devices, which quickens the rising of light emission without increasing power consumption.